High and low signal frequencies, also known as treble and bass signal frequencies respectively, are replicated as sound based upon the electrical signal received by the left and right speakers, as output by a stereo amplifier in a typical configuration. The left and right speakers respond to both the high treble frequencies and the low bass frequencies. Because the left and right speakers are required to produce such a broad spectrum of frequencies, it is common that many loudspeakers fail to produce the lower bass frequencies in a desirable manner. Stated another way, it is not uncommon for bass frequencies to lose their dynamic effect when the left or right speaker is called upon to output sound covering such a broad frequency spectrum. Thus, it is well known that an additional third speaker, known as a subwoofer, may be added to reproduce the lower bass frequencies to emphasize the bass in the reproduced sound.
A typical output from a stereo amplifier includes left and right channel outputs that are electrically coupled to a pair of loudspeakers for reproducing sound corresponding to signals amplified by the stereo amplifier. To create these left and right output channels, a typical amplifier includes two separate power amplifier channels—one for the left output and the other for the right output. Thus, stereo amplifiers that are not equipped with a third powered amplifier channel (which is generally more expensive than a two-channel amplifier) generally cannot be implemented with a subwoofer.
Attempts have been made with varied degrees of success to integrate a subwoofer with a stereo amplifier having two output channels. One possibility for a two-channel stereo amplifier to power a subwoofer is simply to add a third amplifier channel. This option, however, is generally impracticable, since it involves essentially changing the configuration of the stereo amplifier to support the third amplifier channel. It is much more cost-efficient if the third output channel is incorporated in the stereo amplifier at the manufacturing stage, so any post manufacturing modification in this regard is inefficient and costly.
Other potential solutions attempt to capitalize on the fact that typical prerecorded music has bass that is present in both channels equally. With this common practice, it is possible to use a single subwoofer for both channels.
Accordingly, one possibility is to configure a two-channel stereo amplifier so that it supports a third speaker subwoofer through the implementation of a passive crossover circuit. Crossover circuits are commonly used with speakers because no one speaker can produce the entire audio spectrum alone. Tweeter speakers are not configured to produce deep bass, and subwoofers are not configured to produce vocals and other high frequencies with any clarity. This passive crossover solution involves two steps. First, the input of one of the channels has its signal inverted with respect to the remaining channel, and the corresponding stereo speaker connection to this same channel is also inverted. Second, this solution involves placing a subwoofer in a bridge passive crossover connection to the two amplifier channel's “hot” (plus (+) or positive) output terminals with an inductor positioned in series between one of the amplifier's output terminals and the subwoofer input. This inductor placement is known as a passive first order crossover. The passive first order crossover circuit may be implemented so that it does not sum the left and right channel bass information, which, as stated above, is commonly present in both left and right channels equally.
The inductor is simply a coil of wire that typically is wrapped around a permeable ferrite or other type of iron core. The effect of the permeable core is that the inductance of the coil increases in direct proportionality to the permeability of the core as compared to an air core. Thus, an inductor can be made significantly smaller in physical size by incorporating a permeable core. The inductor has the electrical characteristic that its impedance increases proportionally to the frequency of the incoming signal. This increased impedance results in a decrease in stereo amplifier's output, which means that higher frequencies are attenuated at a rate determined by the slope of the crossover filter. Thus, the inductor creates frequency-dependent impedance and operates to roll off the high frequencies so that the high frequencies are not “seen” by the subwoofer.
A passive crossover circuit composed of a simple series inductor, however, cannot remove all of the unwanted frequencies, but it can reduce the output, or roll off, those frequencies to inaudible levels. The passive crossover rolls off the frequencies above a preset frequency cut-off so that just the deep bass frequencies, which may be, for example, 100 Hz and less, are passed to the subwoofer. The frequency cut-off is generally set so that the subwoofer does not attempt to reproduce the higher frequency signals (i.e., frequencies above 100 Hz), as the subwoofer is neither designed nor capable of responding to the demands of higher frequencies.
The problem with this configuration is that even with placing the inductor in series between the subwoofer and one of the channel outputs, an insufficient amount of the midrange frequencies are removed from the output signal prior to reaching the subwoofer. This situation occurs because the passive crossover circuit is a first order crossover, which reduces unwanted frequencies by approximately 6 dB per octave.
The passive crossover circuit may be configured as a second order filter with the addition of a shunt capacitor positioned across the subwoofer's input in an effort to increase the rate of roll off of higher frequency signals. However, due to the addition of the inductor and/or capacitor elements, the cost of these configurations is greater and therefore not desirable. Additionally, some amplifier power is lost in the finite resistance of the inductor wire, the equivalent series resistance, and dielectric absorption of the capacitor.
As another possibility, a dual voice coil subwoofer speaker, configured such that both voice coils drive a common cone in straight polarity, may be used with two passive crossover circuits in similar fashion as described above. A pair of passive crossover circuits in conjunction with a dual voice coil subwoofer may be configured to split the low-frequency audio spectrum so that the left and right channel outputs are each communicated to a separate voice coil on the subwoofer for reproducing the low-frequency sounds. More specifically, a signal corresponding to the left channel bass frequencies is communicated to a first voice coil on the subwoofer, and a signal corresponding to the right channel bass frequencies is communicated to a second voice coil on the subwoofer. The effect of the two separate signals on the two separate voice coils is that the subwoofer cone responds to each voice coil to produce low-frequency sound.
The disadvantage of this implementation is a loss of amplifier power recognized by the subwoofer. In order to roll off the higher frequencies, two inductors are utilized—one coupled between each output of the stereo amplifier and its respective connection to the subwoofer. Even with low resistance inductor coils, there is some volume reduction. This loss is due primarily to the size and the number of wire windings of each inductor. As a result, this is an inefficient solution.
This dual voice coil subwoofer speaker itself is also more costly due to the dual coil windings on the two voice coils and the complex manufacturing involved. Single voice coil speakers are more common and thereby less expensive than dual voice coil speakers.
Another possibility involves directly connecting the subwoofer to both stereo amplifier channel outputs dual-primary winding, high-current, mixing transformer positioned in the path prior to the subwoofer. The mixing transformer operates to passively sum the bass in each channel. However, the effect of implementing the transformer compromises subwoofer performance. Moreover, the cost of the mixing transformer is undesirably high due in part to the size of the transformer for proper configuration. Thus, for this reason, this alternative solution is also inefficient and undesirably expensive.
Thus, a heretofore unaddressed need exists to address the deficiencies and problems described above.